In the surface-mount technology (SMT) portion of the electronic assembly industry, printed circuit boards are made by attaching electronic components or parts, such as integrated circuits, capacitors, etc., to a surface of a printed circuit board. These elements are affixed to the printed circuit board in a number of ways, but in particular, by placing dots or deposits of a cream solder material onto metal pads (also known as lands) formed on a printed circuit board. This is often accomplished through the use of a so-called solder screen printer.
Screen printers are well known in the art and an example of such a screen printer is shown in U.S. Re. Pat. No. 34,615, entitled, “Video Probe Aligning of Object to be Acted Upon,” which is incorporated herein by reference. Also incorporated herein by reference is U.S. Pat. No. 5,807,606, entitled, “Applying Adhesives to Substrates,” and U.S. Pat. No. 5,669,970 entitled “Apparatus for Applying Solder Paste.” These patents illustrate and describe a system by which printed circuit boards are, one by one, introduced into a work station. The printed circuit boards, as mentioned above, have formed upon them a number of metallic pads or lands on a surface. These pads or lands are the positions on the printed circuit board that will have affixed to them the various electrical components mentioned above. Usually, a conductive, usually metallic, substance such as solder cream is used for this purpose, although it is well known to affix such parts to pads or lands with non-metallic adhesives as well.
As smaller printed circuit boards, particularly printed circuit boards for such items as small cell phones, digital cameras and personal digital assistants (PDAs) become widespread, a typical printed circuit board with dimensions of about 64 by 64 cm (approximately 25 by 25 inches) after cutting may be subdivided into a plurality of separate circuit board patterns as small as about 2.5 by 7.6 cm (1 by 3 inches) each. This is known as panelization. In the printed circuit board manufacturing process, once the electronic components have been assembled onto the printed circuit boards, the plurality of printed circuit boards will be cut into individual, smaller-sized circuit boards that will eventually find their way into such electronic equipment as noted above.
With the passage of time, SMT has become more sophisticated and more and more components can be placed on a circuit board of a given size, thus increasing component density. The primary method used today in the SMT industry for solder deposition onto pads or lands on a circuit board is a stencil or screen that is usually made of a thin material, often metallic, in which apertures have been cut, either by chemical etching, physical machining or laser cutting, through the thin material. Electroforming is another form of stencil fabrication where apertures are formed by “growing” or plating metallic material around developed photoresist that is later removed resulting in openings.
The apertures formed in the stencil will match with the pads or lands on the printed circuit board when the printed circuit board and the stencil are brought together plane to plane, with the stencil on top of the board. If one were to look through the apertures in the stencil when it is placed in contact with the top of a printed circuit board, the observer would see the metallic pads or lands immediately below each of the apertures (if the cutting of the apertures was performed correctly). It is evident that, if an aperture is formed by mistake in a place where there is no target pad or land, then often the entire stencil must be discarded as it is difficult to repair such stencils due to the nature of their structure. In addition, as printed circuit board designers change a particular component that is to be placed on the printed circuit board, the size and/or location of the pads and lands will also be changed on the printed circuit board. Changes must also be made to the apertures in the stencil to conform to the new part or new size requirements. These changes, of course, require the manufacture of a new stencil; and unnecessary wastage can be avoided or dramatically reduced if the new stencil to be made is manufactured with apertures in the correct location, dimensions and shape. Today, the well-known Gerber file is a software-based representation of the X and Y location of each of the pads for each of the parts on a particular circuit board, together with the shape, size, orientation and centroid of each pad. There may be hundreds and even thousands of parts on a typical circuit board.
In operation, once the stencil has been prepared, a solder material, usually in the form of a viscous solder “cream,” is swiped across the top surface of the stencil when the stencil is placed in contact with and on top of the printed circuit board. Solder cream is deposited in those apertures that have been formed in the stencil; and, when the stencil is lifted from the printed circuit board, each of the pads or lands will contain a minute, measured amount of solder. It is therefore important, in order to meet manufacturing restrictions (particularly as components and printed circuit boards are reduced in size), that there be a perfect or near perfect alignment of apertures and pads or lands. Additionally, any redesign of the printed circuit board that requires a redesigned stencil should be done efficiently and cost-effectively. To prepare a stencil and to cut apertures into the stencil to match the printed circuit board pattern of pads and lands is a costly operation, and the cost of an apertured stencil can be as high as $1,500. Obviously, if a mistake is made and an aperture is made in the “wrong” place or the aperture is of the wrong shape or size, the stencil may have to be discarded and another one manufactured. Presently, this is a time-consuming, often manual, task.
Today, existing computer hardware and computer software attempt to automate the process of the manufacture of the apertures in a stencil. For example, assemblers of printed circuit boards may provide to the stencil manufacturer the size, shape and location of pads or lands on printed circuit boards in the form of computer-aided-drafting (CAD) data (e.g., Gerber). This CAD data can then be input into the computer system of the stencil manufacturer; assuming the data is correct, the computer system will then guide the stencil manufacturer to form apertures in the stencil in accordance with the pad and land data.
However, a significant problem is faced when, as mentioned earlier, a printed circuit board manufacturer wants to change that data by changing the size, shape or location of the pads. It should also be noted that it is relatively common in printed circuit board patterns to have a number of different components or other parts on a single printed circuit board. Typically, there are multiple instances of a particular pad pattern formed on a single circuit board. At the present time, when a manufacturer or assembler customer determines to change, for example, the size or shape of the pad of a capacitor (which is generally simply a two-pad contact), the board designer must ensure that the pad changes are made for each and every capacitor (of which there may be dozens) on the printed circuit board. The customer does not usually specify the location on the board where changes are to be implemented. Similarly, the apertures in the stencil must match each of the pad changes. This matching process is presently done manually and relies upon the skill of the stencil manufacturer's technicians to find each and every instance where a change must be made. As mentioned earlier, there may be multiple printed circuit board patterns on each stencil and thus the technician must look for and change perhaps hundreds of pad patterns on the stencil. Otherwise, if the technician fails to observe even one capacitor that is being changed, the entire stencil may be incorrect; and the error may not even be realized until the manufacturing process has begun, and defective printed circuit boards that do not have solder deposited on all the pads may halt or significantly slow the printed circuit board manufacturing process. This is costly and to be avoided.
There are a number of software products commercially available that will translate printed circuit board manufacturers' data into stencil-aperturing data. One such example of this is the TRILOGY 5000 software product manufactured by Valor Computerized Systems, Limited of Eyavne, Israel. The TRILOGY 5000 product is described in the Valor TRILOGY Software Manual, Version 6.0, available from Valor Computerized Systems. The TRILOGY software is available to operate on a Personal Computer running Windows, as well as on, for example, a Sun workstation running UNIX. Thus, it would be desirable to have a non-manual/automated system that would allow for an efficient and cost effective redesign of stencil aperturing in a manner that eliminates, or substantially eliminates, human error and that integrates with the present systems for aperture design, such as the TRILOGY 5000 product, to allow for efficient initial design of stencils, as well as modification of such stencils as printed circuit board manufacturers make changes in such boards.
In addition to the need to quickly and accurately create aperture data that aligns with the landing pads on the PCB, precise alignment of the pad and aperture is important. If an aperture is present for each pad but not precisely aligned with it, a material deposit will be made entirely or partially off the pad that results in component-to-PCB connection problems later in the assembly process. Misalignment of the aperture and pad have several primary causes including: variables in the PCB manufacturing process, tolerances in the printing equipment alignment systems, variations inherent to the various stencil fabrication processes, changes that occur in pad/mask definition after initial reflow on 2-sided SMT PCBs, etc. Assemblers have historically compensated for the variables by shrinking the size or changing the shape of the aperture to increase the likelihood that most or all of the aperture will align with the pad.